Seung Ryong Ok and D ieter W. Gruenwedel Department of Food Science and Technology, 109 Food Science Building, University of California, Davis, CA 95616, U.S.A. Z. Naturforsch. 48c, 488-494 (1993); received September 18, 1992/February 2, 1993 Circular Dichroism, Endonucleases, Polynucleotides, Mercury Complexes, Right«-*Left Inversions The long-wavelength positive CD bands of poly[d(A) • d(T)] and poly[d(A-T)-d(A-T)] be come inverted upon the addition of Hg(C104)2. Poly[d(A)-d(T)] requires higher levels of mer cury to undergo inversion than poly[d(A-T)-d(A-T)]\ Mercurated poly[d(A) d(T)] is digested more rapidly than the control by DNase I or staphylococcal nuclease at low levels of Hg(C104)2. Let r = [Hg(C104)2]added/[DNA-P]. A 4to 5-fold rate increase occurs with DNase I at r = 0.25; a 2-fold increase with staphylococcal nuclease at r = 0.2. By contrast, digestion of poly[d(A-T) d(A-T)] decreases immediately with increasing r. The noted rate increases appear to be due to a modification of poly[d(A)-d(T)] helix structure prior to the chiroptical conver sion. The modification is interpreted as a widening of the minor groove, permitting, thus, a better binding of DNase I to its substrate. The overall changes in CD as well as enzymatic digestion rates are taken to signal mercury-induced alterations in helix screwness from rightto-left. They are totally reversible subsequent to the removal o f mercury.